1. Field of the Invention
The application relates to a method in a wireless communication system, and more particularly, to a method of handling a random access procedure on a Primary Cell for a mobile device when a random access procedure on a Secondary Cell is ongoing or about to start.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network including a mobility management entity (MME), serving gateway, etc for NAS (Non Access Stratum) control.
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system, considering relaying for cost-effective throughput enhancement and coverage extension. For example, a relay can be deployed at the cell edge where the eNB is unable to provide required radio quality/throughput for the UEs or at certain location where radio signals of the eNB cannot cover. The LTE-A system can support a wider bandwidth up to 100 MHz to satisfy requirement for peak data rate. Carrier aggregation (CA) where two or more component carriers are aggregated is employed for the LTE-A system to achieve wider-band transmission. An LTE-A specification supports carrier aggregation for both continuous and non-continuous component carrier (CC) with each component carrier limited to a maximum of 110 resource blocks. The carrier aggregation increases spectrum flexibility by aggregating the component carriers in the different frequency band (non-continuous spectrum).
When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment, one serving cell provides the security input (one ECGI, one PCI and one ARFCN) and the NAS mobility information (e.g. TAI). This cell is referred to as the Primary Cell (PCell). Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. The configured set of serving cells for a UE always consists of one PCell and one or more SCells.
A UE shall not perform any uplink transmission except the Random Access (RA) Preamble transmission if its uplink transmission timing is unsynchronized. A Random Access procedure is used to achieve uplink time synchronization for a UE which either has not yet acquired, or has lost, its uplink synchronization. The random access procedure comes in two forms, contention-based or non contention-based. In a contention-based random access procedure, a random access preamble signature is randomly chosen by the UE, with the result that it is possible for more than one UE simultaneously to transmit the same signature, leading to a need for a subsequent contention resolution process. Contention Resolution is based on either C-RNTI on PDCCH of the PCell or UE Contention Resolution Identity on a downlink share channel (DL-SCH). For the non contention-based random access procedure, the eNodeB has the option of preventing contention from occurring by allocating a dedicated signature to a UE, resulting in contention free access.
FIGS. 1A and 1B are diagrams showing two kinds of random access procedures in the prior art. As seen in FIG. 1A, the three steps of non-contention based random access procedures are: random access preamble assignment via dedicated signaling in downlink; random access preamble on random access channel in uplink; random access response on downlink shared channel (DL-SCH). The E-UTRAN needs to transmit an RA response (RAR) corresponding to the RA preamble to the UE. A MAC RAR usually consists of three fields: TA Command (Timing Advance Command)/UL Grant/Temporary C-RNTI. In FIG. 1B, the four steps of the contention based random access procedures are: random access Preamble on RACH in uplink; random access response generated by eNB on DL-SCH; first scheduled uplink transmission on uplink shared channel (UL-SCH); Contention Resolution on downlink. The UE can perform either contention-based or non contention-based random access procedure on PCell, but the UE can only perform non contention-based random access procedure on SCell.
So far, the UE does not need to support parallel random access procedures in order to avoid increasing the complexity in the UE implementation. However, if a random access procedure on a SCell triggered by need of SCell synchronization (e.g. triggered by a PDCCH order received on the SCell or on a scheduling cell of the SCell) is ongoing or is about to start, meanwhile, the UE also tries to initiate random access procedure on a PCell triggered by SR. In some situations, the UE may trigger an SR. When an SR is triggered, it shall be considered as pending until it is cancelled. As long as one SR is pending, for each transmission time interval (TTI), if the UE has no available uplink share channel (UL-SCH) resources in this TTI and the UE has no valid PUCCH resource for SR in any TTI, the UE will initiate a random access procedure and cancel all pending SRs. All pending SR(s) shall be cancelled when a MAC protocol data unit (PDU) is assembled and this PDU includes a buffer status report (BSR) which contains buffer status up to (and including) the last event that triggered a BSR, or when the UL grant(s) can accommodate all pending data available for transmission. Similarly, the UE may be requested by an eNB to perform a random access procedure during initiating another random access procedure on a PCell triggered by SR. In above cases, the UE will perform parallel random access procedures.